Look at this first pic -- only pic I had was with the rod out, but you can see where it goes in the center of the sender. That sounds like what you have. Won't work.
What you need is a gear over it as shown in the second pic. There's actually a little clip that's needed to keep the gear in place, but it's not in the pic.
That could/should be the solution. Call any of the Mustang places and get a gear to fit the Tremec. I believe the same one from a T5 would work.
Cheers,
-Neil.
